1. Field of the Invention
The present invention relates to a motor. More particularly, the present invention relates to a structure for preventing electrochemical corrosion of a bearing in a motor.
2. Description of the Related Art
In motors driven by a higher voltage of 100 volts or higher and controlled by PWM (pulse width modulation) control, an axial electric current caused by a potential difference in the motor has been a problem for some time. The potential difference is produced by an unbalanced voltage applied to the motor, unevenness of a gap between a stator and a magnet, or a voltage change at a neutral point in PWM control, for example. That axial electric current causes an electric discharge in a bearing in which an electrically insulating layer exists, e.g., a roller bearing in which each of inner and outer rings is provided with an electrically insulating layer. The electric discharge roughens a bearing surface, especially a rolling surface of a roller bearing or a surface of a ball. Such a rough surface plays a role in causing abnormal noise.
FIG. 7 shows a structure of an exemplary conventional motor driven at a high voltage. FIG. 7 is a cross-sectional view of the motor taken along a plane containing its axial direction. FIG. 8 is a plan view of a bearing of the motor, showing the inner structure thereof.
Referring to FIG. 7, the motor 1 includes a shaft 2 rotatable about a rotation axis J1, a rotor 3 secured to an axially central portion of the shaft 2, a stator 4 arranged outside the rotor 3 in a radial direction perpendicular to the rotation axis J1 with a gap therebetween, roller bearings 5 spaced axially away from each other and sandwiching the rotor 3 therebetween, and a frame 6 including a hollow and cylindrical portion 6a which secures the stator 4 thereto. The roller bearings 5 are retained by the frame 6. Referring to FIG. 8, each roller bearing 5 includes an inner ring 5a and an outer ring 5b. The inner ring 5a is secured to the shaft 2 and can rotate together with the shaft 2. The outer ring 5b is secured to the frame 6 and does not rotate. A plurality of balls 5c are in contact with both the inner and outer rings 5a and 5b. The balls 5c can rotate and revolve around the rotation axis J1. A space between the inner and outer rings 5a and 5b is filled with grease 5d. The roller bearing 5 further includes a retainer for retaining a circumferential gap between the balls 5c, although it is not shown in FIG. 8.
In the motor shown in FIG. 7, an axial electric current flows mainly through a path containing the frame 6, the roller bearing 5, and the shaft 2 in that order, as shown with dashed arrow. Referring to FIG. 8, when the axial electric current flows between the inner ring 5a and the outer ring 5b of the roller bearing 5, a spark occurs because the grease 5d is made of electrically insulating material. This spark roughens surfaces of the inner ring 5a and outer ring 5b, which are opposed to the balls 5c, and surfaces of the ball. That is, the roller bearing 5 is electrochemically corroded. Thus, an abnormal noise is generated during the operation of the roller bearing 5 in this state, and a life of the roller bearing 5 is shortened.